Polymerizing hydrogels including modifying compounds to comprise low amount of residual monomers and by-products and to optimize material properties

ABSTRACT

The present invention relates to polymerized hydrogels and processes to make such hydrogels, in particular hydrogel adhesives which are capable of attaching to mammalian skin and can be used in various personal care products, such as waste-management articles, and a variety of functional articles to be worn by a human. The hydrogels described herein are characterized by very low amount of residual starting monomers, impurities, and/or by-products that could be formed during polymerization. Specifically, the hydrogels are made by adding scavengers and/or chain transfer agent prior to polymerization. It has been found, that upon addition of same scavengers the material properties of the polymerized hydrogel differ from the properties of gels polymerized without the scavenger. This is due to the fact, that these specific scavengers act also as chain transfer agents in the radical polymerization. Further studies showed that also chain transfer agents, that are no scanvengers for residual monomer(s), impurities or byproducts influence the material properties of the polymerized hydrogel adhesive.

FIELD OF THE INVENTION

The present invention relates to polymerized hydrogels and processes tomake such hydrogels, in particular hydrogel adhesives which are capableof attaching to mammalian skin and can be used in various personal careproducts, such as waste-management articles, and a variety of functionalarticles to be worn by a human. The hydrogels described herein arecharacterized by very low amount of residual starting monomers,impurities, and/or by-products that could be formed duringpolymerization. Specifically, the hydrogels are made by addingscavengers and/or chain transfer agent prior to polymerization.

It has been found, that upon addition of same scavengers the materialproperties of the polymerized hydrogel differ from the properties ofgels polymerized without the scavenger. This is due to the fact, thatthese specific scavengers act also as chain transfer agents in theradical polymerization.

Further studies showed that also chain transfer agents, that are noscavengers for residual monomer(s), impurities or byproducts influencethe material properties of the polymerized hydrogel adhesive.

The method of adding chain transfer agents prior to polymerization canbe used to easily optimize the material properties of a hydrogeladhesive.

BACKGROUND OF THE INVENTION

While adhesive materials, e.g. hydrogels, in particular mammalian skinadhesives for use in consumer products such as absorbent articles andwaste-management articles have previously been described in EP 1 025 823and EP 1 025 866 respectively, the disclosure of these adhesivematerials has mainly occurred in the context of different medicalapplications, such as skin electrodes, transdermal drug delivery andwound healing respectively. Certain hydrogel requirements for consumerproducts produced on a large scale, such as absorbent and humanwaste-management products, are disclosed in EP 1 025 823 and EP 1 025866. Herein the need for secure attachment, stability of adhesion inpresence of excess moisture, and painless removal are included.

Additionally it is particularly important to delivering theabove-mentioned benefits, that the hydrogel used must provide a verygood safety profile, especially for large scale production of consumerproducts.

It has been discovered that complete conversion of the used monomers,especially of acrylic acid and derivatives was impossible when lowmolecular-weight water-soluble and high-molecular weight polymers andcopolymers that are soluble or swell up in water (partly crosslinked)had to be prepared. Residual contents above 0.5 and even 1.0% of freemonomers are often found in polymers manufactured on an industrialscale.

Since it has been impossible up to now to carry out polymerizationwithout leaving residual monomers, attempts have been made to remove theresiduals. This can be achieved either by eliminating the residualmonomers or by converting them into safe derivatives.

In U.S. Pat. No. 4,132,844 a method is mentioned for directly reducingthe amount of free monomers in an aqueous polymer gel by heating saidpolymer at a high temperature. In Japanese Patents Nos. 53/51289 and50/136382, residual monomer content has been reduced by extraction withsuitable solvents.

U.S. Pat. Nos. 2,960,486, 3,755,280, and 4,929,717 describe thetreatment of a polymer gel based on acrylic acid and/or acrylamide whichwas made in a conventional manner, with different compounds. The treatedpolymer gel is then subsequently and systematically dried at an elevatedtemperature after this treatment before any residual monomer contentanalysis was carried out.

Unfortunately not only the level of starting unreacted monomers, butalso the level of impurities and by-products that could arise from thepolymerization step such as acrolein, acrylonitrile or acrylamide, hasto be controlled and kept within specifically defined target levels inthe resulting hydrogel composition.

None of the above-cited cases were concerned in reducing impuritiesand/or by-products that could be produced during the polymerization stepof starting monomers.

The present invention provides a process for making polymerizedhydrogels with very low amount of residual starting monomers, impuritiesand/or any by-products that could be produced during the polymerizationstep and/or adjusted properties. This polymerization being conductedfrom within a reaction medium comprising from 10-90 wt % water, from10-60 wt % of starting monomers and from 10-80 wt % of a polyol.

The process described in the present invention consists in twosuccessive steps. The first step is a treatment of the polymerizablepremix solution with chain transfer agents and/or compounds that reactwith residual monomers, Impurities and by-products that could be formedduring the polymerization step. The second step is the polymerization ofthe so treated monomer solution leading to an extremely low content ofresidual monomers and impurities respectively and/or adjusted propertiesas tan δ₂₅.

It is known that when polyols, e.g. glycerol and the like, are presentin polymerized hydrogel made by UV initiation, the level of acroleinmust be controlled in the finished composition, and be kept underwell-defined target levels. Indeed, contact with acrolein is preferablyavoided or should be minimized.

It has also been found that by controlling the pH of the monomer pre-mixsolution, the level of acrolein formed during the polymerizationreaction is reduced. Furthermore, it has been described that bycarefully controlling the UV-radiation during the photopolymerizationreaction, it is possible to reduce the formation of acrolein viaphotodecomposition of free-radical reactions involving glycerol.

It is one purpose of the present invention to provide a method formaking polymerized hydrogel with very low level of residual monomers andor other impurities. It is especially useful to reduce the level ofcompounds that carry carbonylic groups and α,β-unsaturated carbonylicfunctionalities. The process as claimed, comprises a step consisting intreating monomer premix solutions directly before polymerization, tothereby reduce the concentration of acrolein below long-term safetylevels. The present invention is also efficient for reducing the levelsof other impurities or by-products including acrylonitrile, acrylamideand residual monomers respectively.

While U.S. Pat. No. 5,606,094 describes a process for scavengingacrolein from a gaseous or liquid mixture containing acrolein inacrylonitrile with sodium bisulfite followed by separation of thereaction products, the process described in the present inventionprovides a method for incorporating the impurity scavenger before thepolymerization step. Therefore the mentioned side products are reducedimmediately in the time of their formation. In addition to that residualmonomers are reduced by the reaction with surplus of the scavengercompound which can be e.g. sodium bisulfite or any hetero nucleophile.

Another purpose of the present invention is to optimize the materialproperties of the hydrogel adhesive by adding chain transfer agentsprior to the polymerization.

SUMMARY OF THE INVENTION

In one embodiment, the present invention relates to a process for makingpolymerized hydrogels, in particular hydrogel adhesives, comprising10-90 wt % water and 10-60 wt % of a cross-linked hydrophilic polymer.The hydrophilic polymer is made by polymerizing of at least one startingmonomer type, and contains 580 wt %, preferably 10-80 wt %, mostpreferably 30-80 wt % of at least one polyol.

The process described in the present invention consists in twosuccessive steps. The first one consists in mixing said startingmonomer(s) within a reaction medium comprising from 10-90 wt % water,from 10-60 wt % of said starting monomer(s) and from 10-80 wt % of atleast one polyol, to thereby form a polymerizable monomer solution. Tothis solution is added a modifying compound pure or in solution andoptionally mixed well carefully avoiding the polymerization reaction totake place. In addition an early reaction of the polymerizable monomerswith the scavenger compound has to be avoided. The modifying compoundcan be one chemical entity or a mixture of chemical entities with thesame or different effects on the hydrogel. The modifying compound isselected from the group consisting of scavenger compound, chain transferagent and compound which is a scavenger compound and chain transferagent.

The second step consists in polymerizing the reaction mixture formed inthe first step, to form an hydrogel material. While the polymerizationreaction takes place, the scavenger compound immediately reacts withresidual monomer(s), impurity(s) and/or with any by-products produced bysaid polymerization reaction, to thereby reduce the concentration ofsaid residual starting monomer(s), impurity(s) and/or said by-product(s)within said hydrogel.

In a preferred embodiment, the present invention relates to a processallowing to obtaining polymerized hydrogel, in particular adhesive,wherein the polymerization is carried out at least partly by UVirradiation.

The pH of the hydrogel ranges from pH 3.5 to 7, preferably 4 to 6.5,more preferably 4.5 to 6.

In another embodiment, the present invention relates to polymerizedhydrogel, in particular adhesive, comprising 10-90 wt % water, 10-60 wt% of cross-linked hydrophilic polymer made from starting monomer(s), and10-80 wt % of at least one polyol, such hydrogel being prepared bypolymerizing said starting monomer(s) in the presence of said water andpolyol(s), wherein such hydrogels contain less than 100 ppb, preferablyless than 50 ppb, and most preferably less than 20 ppb ofa,b-unsaturated carbonyl by-product(s) derived from said polyol(s)during polymerization, and wherein the level of residual startingmonomer(s) is below 200 ppm, preferably below 100 ppm, more preferablybelow 50 ppm, even more preferably below 20 ppm, and most preferablybelow 10 ppm.

In still another embodiment, the present invention relates topolymerized hydrogel, in particular adhesive, comprising 10-90 wt %water, 10-60 wt % of cross-linked hydrophilic polymer made from startingmonomer(s), and 10-80 wt % of at least one polyol, such hydrogel beingprepared by polymerizing said starting monomer(s) in the presence ofsaid water and polyol(s), wherein such hydrogels comprise more than 20ppb, preferably more than 50 ppb, more preferably more than 100 ppb,even more preferably more than 500 ppb, and most preferably more than1000 ppb of nucleophilic addition product(s) of the α,β-unsaturatedcarbonyl by-product(s) derived from said polyol(s) duringpolymerization.

In a further embodiment, the present invention relates to polymerizedhydrogel, in particular adhesive, comprising 10-90 wt % water, 10-60 wt% of cross-linked hydrophilic polymer made from starting monomer(s), and10-80 wt % of at least one polyol, such hydrogel being prepared bypolymerizing said starting monomer(s) in the presence of said water andpolyol(s), wherein such hydrogels are characterized by having a tan δ₂₅above 1.

DETAILED DESCRIPTION

The present invention relates to polymerized hydrogels and processes tomake such hydrogels, in particular hydrogel adhesives, which are capableof attaching to mammalian skin.

In a first embodiment, the present invention relates to a process formaking a hydrogel comprising 10-90 wt % water, 10-60 wt % ofcross-linked hydrophilic polymer made from at least one starting monomertype, and 10-80 wt % of at least one polyol. This process comprises afirst step consisting in preparing said monomer(s) solution from 10-90wt % water, from 10-60 wt % of said starting monomer(s) and from 5-80 wt%, preferably 10-80 wt %, most preferably 30-80 wt % of said polyol(s),and adding a modifying compound to and optionally mixing well in themonomer solution prior to polymerization of the so formed mixture. Apart of the amount of the modifying compound can also be added after thepolymerization.

In the process of the present invention, the compound which reacts withthe starting monomers, impurities, and/or by-products mentioned belowand/or the chain transfer agent is preferably added directly to themonomer premix solution in a stirring vessel, a tube or a static mixerand the like. The compound can be added as a pure substance or asmixture of substances or in solution, preferably in aqueous solution andalso preferably the quantity of added solution is sufficiently lowrelative to the amount of the monomer premix solution such that it canbe rapidly mixed in the reaction mixture. Alternatively the reactionmixture can be stored by low temperature, e.g. 10° C. or can bestabilized by known polymerization inhibitors.

In a second step the so formed reaction mixture is polymerized tothereby form a hydrogel. In preparing hydrogels in accordance with thepresent invention, the ingredients will usually be mixed to provide areaction mixture in the form of an initial pre-gel aqueous based liquidformulation, in this case treated with the modifying compound, which isthen converted into a gel by a free radical polymerization reaction.This may be achieved for example using conventional thermal initiators,redox initiators and/or photoinitiators or by ionizing radiation. Suchfree-radical polymerization initiators are well known in the art and canbe present in quantities up to 5% by weight, preferably from 0.02% to2%, more preferably from 0.02% to 0.4%. Photoinitiation is a preferredmethod and will usually be applied by subjecting the pre-gel reactionmixture containing an appropriate photoinitiation agent to UV lightafter it has been spread or coated as a layer on silicone-coated releasepaper or other solid or porous substrate.

For use in forming the homopolymer or co-polymer component of thepolymerized hydrogel, suitable monomers or co-monomers can be acidic,neutral, basic, or zwitterionic. Among acidic monomers, suitablestrong-acid types include those selected from the group of olefinicallyunsaturated aliphatic or aromatic sulfonic acids such as 3-sulfopropyl(meth) acrylate, 2-sulfoethyl (meth) acrylate, vinylsulfonic acid,styrene sulfonic acid, allyl sulfonic acid, vinyl toluene sulfonic acid,methacrylic sulfonic acid and the like and the respective salts.Particularly preferred strong-acid type monomer is2-acrylamido-2-methylpropanesulfonic acid and its salts. Among acidicmonomers, suitable weak-acid types include those selected from the groupof olefinically unsaturated carboxylic acids and carboxylic acidanhydrides such as acrylic acid, methacrylic acid, maleic acid, itaconicacid, crotonic acid, ethacrylic acid, citroconic acid, fumaric acid andthe like and the respective salts. Particularly preferred weak-acid typemonomer is acrylic acid and its salts.

Examples of neutral monomers include N,N-dimethylacrylamide, acrylamide,N-isopropyl acrylamide, hydroxyethyl (meth)acrylate, alkyl(meth)acrylates, N-vinyl pyrrolidone and the like. Examples of cationicmonomers include N,N-dimethylaminoethyl (meth)acrylate,N,N-dimethylaminoethyl (meth)acrylamide and the respective quaternarysalts and the like. Most preferably, the hydrogel compositions of theinvention are based upon acrylic acid monomer and its salts.

The cross-linking between polymer chains creates a 3-dimensional matrixfor the polymer, also referred to as gel form or hydrogel. Physicalcross-linking refers to polymers having crosslinks that are not chemicalcovalent bonds but are of a physical nature such that for example thereare areas in the 3 dimensional matrix having high crystallinity or areashaving a high glass transition temperature or areas having hydrophobicinteractions. Chemical cross linking refers to polymers which are linkedby covalent chemical bonds, The polymer can be chemically cross linkedby radiation techniques such as UV, E beam, gamma or micro-waveradiation or by co-polymerizing the monomers with a di/polyfunctionalcrosslinker via the use e.g., of UV, thermal and/or redox polymerizationinitiators. The polymer can also be ionically crosslinked.

Suitable polyfunctional monomer crosslinkers include polyethyleneoxidedi(meth)acrylates with varying PEG molecular weights, IRR280 (a PEGdiacrylate available from UCB Chemical), trimethylolpropane ethyoxylatetri(methacrylate with varying ethyleneoxide molecular weights, IRR210(an alkoxylated triacrylate available from UCB Chemicals),trimethylolpropane tri(meth)acrylate, divinylbenzene, pentaerythritoltriallyl ether, triallylamine, N,N-methylene-bis-acrylamide and otherspolyfunctional monomer crosslinkers known to the art. Preferred monomercrosslinkers include the polyfunctional diacrylates and triacrylates.

Chemical crosslinking can also be effected after polymerization by useof polyfunctional reagents capable of reacting with polymer functionalgroups such as ethyleneglycol diglycidyl ether, polyols such asglycerol, and other polyfunctional reagents known to the art.

Crosslinking can also be effected all or in part by ionic crosslinkingwherein groups of opposite charge interact via ionic interactions.Suitable ionic crosslinking agents include those known to the artincluding polyvalent cations such as Al³⁺ and Ca²⁺, di/polyamines,di/poly-quaternary ammonium compounds, including polymeric polyaminesand quaternary ammonium compounds known to the art.

The hydrogel compositions described herein can comprise a humectant, ormixture of humectants (also referred as a plastisizer), which ispreferably a liquid at room temperature. The humectant is selected suchthat the monomer and polymer may be solubilized or dispersed within. Forembodiments wherein irradiation crosslinking is to be carried out, thehumectant is desirably irradiation crosslinking compatible such that itdoes not significantly inhibit the irradiation crosslinking process ofthe polymer. The components of the humectant mixture are preferablyhydrophilic and miscible with water.

Suitable humectants include alcohols, polyhydric alcohols such asglycerol and sorbitol, and glycols and ether glycols such as mono- ordiethers of polyalkylene glycol, mono- or diester polyalkylene glycols,polyethylene glycols, glycolates, glycerol, sorbitan esters, esters ofcitric and tartaric acid, imidazoline derived amphoteric surfactants.Particularly preferred are polyhydric alcohols such as glycerol andsorbitol, polyethylene glycol, and mixtures thereof. Glycerol isespecially preferred. The humectant comprises 5-80 wt % of the hydrogel.

Other common additives known in the art such as polymerizationinhibitors, chain transfer agents, salts, surfactants, soluble ordispersible polymers, buffers, preservatives, antioxidants, pigments,mineral fillers, and the like and mixtures thereof may also be comprisedwithin the adhesive composition in quantities up to 10% by weight eachrespectively.

The term polyols refer to alcohol compounds having more than onehydroxyl group. Polyols include polyhydric alcohols and are also calledpolyalcohols. As it was mentioned previously, polyols are well known inthe art as common additives for making hydrogels. Therefore, a methodfor reducing by-products formed from these polyols duringpolymerization, is particularly useful.

In a preferred embodiment of the present invention, is provided aprocess where the polymerization is conducted at least partly byphotoinitiation polymerization. Photoinitiation will usually be appliedby subjecting the pre-gel reaction mixture of monomer(s) containing anappropriate photoinitiation agent to UV light after it has been spread,coated, or extruded as a layer on silicone-coated release paper or othersolid or porous substrate. The incident UV intensity, typically at awavelength in the range from about 240 to about 400 nm overlaps to atleast some degree with the UV absorption band of the photoinitiator andis of sufficient intensity and exposure duration (e.g., 120-36000mW/cm²) to complete the polymerization of the reaction mixture.

Such free radical photoinitiation agents or photoinitiators are wellknown in the art and can be present in quantities up to 5% by weight,preferably less than 1%, more preferably less than 0.5%, and mostpreferably less than 0.4%. Such photoinitiators include typeá-hydroxy-ketones and benzilidimethyl-ketals. Suitable photoinitiatorsinclude dimethylbenzylphenone (available under the trade name orIrgacure 651 from Ciba Speciality Chemicals).2-hydroxy-2-methyl-propiophenone (available under the trade name Darocur1173 from Ciba Speciality Chemicals), 1-hydroxycyclohexyl-phenyl ketone(available under the trade name Irgacure 184 from Ciba SpecialityChemicals), diethoxyacetophenone, and4-(2-hydroxyethoxy)phenyl-(2-hydroxy-2-methylpropyl) ketone (availableunder the trade name of Irgacure 2959 from Ciba Speciality Chemicals).Darocure 1173, Irgacure 2959 and Irgacure 184 are preferredphotoinitiators. Irgacure 2959 and Irgacure 184 are particularlypreferred. In the hydrogel compositions described in the presentinvention, Irgacure 2959 is the most preferred photoinitiator.Combinations of photoinitiators can also be used. In addition,polymerization can be carried out by using thermal initiator(s) and/orredox initiator(s) well known to the art or one or more of theseinitiators in combination with the aforementioned photoinitiators.Suitable thermal initiators include potassium persulfate and VA044(available from Wako). Suitable redox initiators include the combinationof hydrogen peroxide and ascorbic acid and sodium persulfate andascorbic acid.

It has been shown that during the photopolymerization process, whenglycerol is used as the polyol, it can produce acrolein as a by-product.A method suitable for measuring the level of acrolein in a polymerizedadhesive hydrogel is described in the Test Methods section.

Without being bound by theory, it is believed that acrolein (2-propenal)can be formed by acid-catalyzed or base-catalyzed reactions of glyceroland glycerol esters with free radicals generated duringphotopolymerization, wherein the concentration of free radicals areespecially high. It is believed that by controlling the pH within thelimits described hereinafter, the amount of acrolein generated duringphoto-polymerization as a result of these acid or base catalyzedreactions can be diminished.

Also, without being bound by theory, it is believed that the analogousreaction(s) can occur with other polyols yielding α,β-unsaturatedcarbonyl by-products such as ene-als, ene-ones and the like.

It has been described, in a co-pendant application, that by controllingthe pH of the monomer pre-mix solution in the range of 3.5 to 7,preferably 4-6.5, more preferably 4.5-6; that the level of acroleinformed during the polymerization reaction is reduced. This is especiallyimportant to control the level of acrolein in the finished hydrogel.

Furthermore, it has been found that the wavelength of the UV-radiationshould be carefully controlled during the photopolymerization reaction,to obtain optimum results on reduction of acrolein. It is preferable tominimize the relative percentage of UV irradiation reaching the monomersolution and hydrogel with wavelengths below 280 nm, preferably below300 nm, more preferably below 320 m, most preferably below 335 nm. Thiscan be achieved by the use of a UV light source that has inherently lowoutput in these wavelength ranges or by interposing one or morehigh-pass UV-filters between the UV light source and the monomersolution and hydrogel.

Examples of high-pass UV filters that can be used for this purposeinclude the Boro-float UV Filters (e.g., T320) available fromBedampfungs-technik. Other examples include the high-pass UV filtersmade by Schott Glass Werke (e.g. WG-280, WG-295, WG-305, WG-320, andWG-325). It is preferred that the integrated UV intensity in units ofW/cm2 in the aforementioned wavelength regions by reduced to less than10%, preferably less than 7%, more preferably less than 4%, mostpreferably less than 1% of the integrated UV intensity in the entireregion (i.e., 200400 nm).

Without being bound by theory, it is also believed that reducing the UVirradiation in the aforementioned wavelength ranges also reduces theformation of acrolein via photodecomposition or fee-radical reactionsinvolving glycerol.

Nevertheless, the preferred overall strategy is to choose polymerizationconditions that reduce the concentration of starting monomers and theirimpurities to very-low levels, even if it generates an increasedconcentration of by-products.

In the case where the polymerization is conducted at least partly by UVirradiation, this step may depend on two process parameters, theincident UV peak intensity (in units of W/cm²) and/or the total UVenergy (in units of J/cm²). It is preferred to use UV irradiation whichleads to a total UVA energy ranging from 0.1-30 J/cm², preferably from0.1-25 J/cm², more preferably from 1-20 J/cm². These conditions arethose preferred at driving down the starting monomer(s).

The process as claimed in the present invention comprises a chemicalpre-polymerization treatment of the monomer premix solution, with acompound that reacts with residual monomers, impurities and/orby-products of the polymerization reaction.

Residual monomers are the unreacted monomers of the hydrophiliccrosslinked polymer of the current invention.

Impurities include conjugated olefins such as acrylonitrile, acrylamide,acrolein, acrylates, t-butylacrylamide, other substituted acrylamidesand the like that are introduced into the hydrogel premix in minoramounts along with the main ingredients. Some conjugated olefins can befound as impurities and also be formed as by-products of thepolymerization reaction.

The chemical treatment refers to any chemical reactions known in the artthat may be applied to a compound. These reactions include, but are notlimited to, substitution, addition, elimination, cyclisation, pericyclicreaction, oxidation, and reduction. Addition reactions are particularlypreferred in the process described in the present invention.

The by-products of the polymerization reaction refer to all productsthat are produced from any ingredients of the reaction medium includingimpurities, whatever the polymerization conditions applied are. Theby-products produced from said polyol(s) are of particular concern inthe present invention.

These by-products may comprise α,β-unsaturated carbonyls such asacrolein, acrylamides, acrylates, and the like. For example, as it waspreviously mentioned glycerol can produce acrolein as a decompositionproduct during the photopolymerization step. It is also known thatacrylamido-2-methane propanesulfonic acid (AMPS) can decompose togenerate acrylamide. Acrolein is the by-product of particular concern inthe present invention. But other by-products that could derive fromcommon additives used for making hydrogels, are within the scope of theinvention.

The scavenger compound that reacts with residual monomers, impurities,and/or by-products can be in particular, a nucleophile, an oxidizingagent, a reducing agent, a conjugated diene or mixtures of these. Forthe process described in the present invention, it is particularlypreferred that the compound be a nucleophile.

Suitable nucleophiles include the whole range of hetero nucleophileswherein hetero nucleophiles are nucleophiles with a polarizableheteroatom like N, S, O or P. Preferred nucleophiles are ammonia,ammonium salts of mineral and carboxylic acids (e.g. chlorides,bromides, sulfates, phosphates, formiates, acetates, acrylates,propionates, tartrates and the like), arylamines (wherein arylpreferably means monocyclic or bicyclic aromatic rings which areoptionally substituted by one, two or more substituents. Thesubstituents are independently of each other preferably selected fromthe group consisting of C₁-C₆-alkyl, OH, C₁-C₆-alkoxy, nitro, halogenetc. Examples are e.g. aniline, methylaniline, benzylaniline, xylidineand the like), heteroaromates (wherein heteroaromates preferably meansmonocyclic or bicyclic aromatic rings with one, two, or more heteroatomslike N, O, S, which are optionally substituted by one, two or moresubstituents. The substituents are independently of each otherpreferably selected from the group consisting of C₁-C₆-alkyl, OH,C₁-C₆-alkoxy, nitro, halogen etc. Preferred are N-heteroaromates.Examples ate e.g. pyridine, imidazole, methylimidazole etc.),alkylamines and/or their mineral or carboxylic salts (alkylamines meanspreferably mono-, di- or trialkylamines with C1-C₆ alkyl chains whereintwo alkyl chains can form together with the N a ring of 5 or 6 members.Examples are e.g., piperidine, piperizine, mono-, di- andtri-butylamine, dimethylamine, diethylamin, dipropaneamine,triethylamine, etc.), multifunctional amines (which are preferablymono-, di- or triamines of alkyl or aryl amines. Examples are e.g.hexamethylendiamine, ethylendiamine, propanediamine diethylentriamine)polyamines (e.g. polyvinylamine), hydroxylamine, hydrazine,aminoguanidine, alkali sulfites, ammonium sulfites, alkali or ammoniumhydrogen sulfites, alkali-, or ammonia-metabisulfites or -bisulfites,hydrogen halide, bromosuccinimide, pyridinium bromide, bromine, orthiols. Aminoguanidine, bisulfite and metabisulfite are particularlypreferred in the present invention.

Oxidizing agents may include permanganate, bichromate, chromate,selenium dioxide, osmium tetroxide, sodium periodate, or ozone,peroxides (sodium persulfate, dibenzoylperoxide etc.) or hydroperoxides(e.g. benzoylhydroperoxide, hydrogenperoxide).

Reducing agents may include metal hydrides, sodium hypochlorite, metalsand their salts of mineral and carboxylic acids (e.g. chlorides,bromides, sulfates, phosphates, formiates, acetates, acrylates,propionates, tartrates and the like), or Grignard reagents, metalchelates (e.g. iron, titanium, cer, cupper, cobald, manganese chelatesof EDTA class of compoundes and derivatives, preferably BASF trilon®)brands), alkali and ammonia sulfites, methane sulfine acids and theirsalts, e.g. sodium formaldehyde sulfoxylate, saccharides (e.g. ascorbicacid, glucose, frutose and the like).

Dienes may include cyclopentadiene, hexachlorocyclopentadiene, isoprene,2-methoxybutadiene, and the like.

When the compound is a nucleophile, it is particularly preferred that itreacts with the double bond(s) of the starting monomers, impuritiesand/or the by-products by an addition reaction.

In the process of the present invention, the scavenger compound whichreacts with said residual starting monomer(s), impurity(s) and/orby-products is preferably present in amounts of less than 30000 ppm,preferably less than 10000 ppm, more preferably less than 5000 ppm orless than 2000 ppm, most preferably less than 1000 ppm, with respect tothe hydrogel. Normally the minimum amount of scavenger compound is morethan 200 ppm, preferably more than 100 ppm, more preferably more than 50ppm, most preferably less than 10 ppm.

The resulting hydrogel contains less than 200 ppm, preferably less than100 ppm, more preferably less than 50 ppm, and even more preferably lessthan 20 ppm, most preferably less than 10 ppm of all residualmonomer(s). Additionally, it is preferred that the resulting hydrogelcontain less than 1000 ppb, preferably less than 500 ppb, morepreferably less than 100 ppb, even more preferably less than 50 ppb, andmost preferably less than 20 ppb of by-product(s) derived from saidpolyol(s) during polymerization. Furthermore, and if applicable, it ispreferred that the polymerized hydrogel contain less than 100 ppb,preferably less than 50 ppb, more preferably less than 25 ppb and mostpreferably less than 10 ppb of acrylonitrile and/or acrylamide.

In another embodiment, the present invention relates to polymerizedhydrogel, in particular adhesive, comprising 10-90 wt % water, 10-60 wt% of cross-linked hydrophilic polymer made from starting monomer(s), and10-80 wt % of at least one polyol, such hydrogel being prepared bypolymerizing said starting monomer(s) in the presence of said water andpolyol(s), wherein such hydrogels contain less than 100 ppb, preferablyless than 50 ppb, and most preferably less than 20 ppb ofα,β-unsaturated carbonyl by-product(s), derived from said polyol(s)during polymerization, and wherein the level of residual startingmonomer(s) is below 200 ppm, preferably below 100 ppm, more preferablybelow 50 ppm, and even more preferably below 20 ppm, and most preferablybelow 10 ppm.

In yet another embodiment, the present invention relates to polymerizedhydrogel, in particular adhesive, comprising 10-90 wt % water, 10-60 wt% of cross-linked hydrophilic polymer made from starting monomer(s), and10-80 wt % of at least one polyol, such hydrogel being prepared bypolymerizing said starting monomer(s) in the presence of said water andpolyol(s), wherein such hydrogels contain less than 100 ppb, preferablyless than 50 ppb, and most preferably less than 20 ppb of acrolein andwherein the level of residual starting monomer(s) is below 200 ppm,preferably below 100 ppm, more preferably below 50 ppm, and even morepreferably below 20 ppm, and most preferably below 10 ppm.

In still another embodiment, the present invention relates topolymerized hydrogel, in particular adhesive; comprising 10-90 wt %water, 10-60 wt % of cross-linked hydrophilic polymer made from startingmonomer(s), and 10-80 wt % of at least one polyol, such hydrogel beingprepared by polymerizing said starting monomer(s) in the presence ofsaid water and polyol(s), wherein such hydrogels comprise more than 20ppb, preferably more than 50 ppb, more preferably more than 100 ppb,even more preferably more than 500 ppb, and most preferably more than1000 ppb of nucleophilic addition product(s) of the a,b-unsaturatedcarbonyl by-product(s) derived from said polyol(s) duringpolymerization. The aforementioned nucleophilic addition product(s)refer to all products resulting directly or indirectly from saidaddition reaction between a suitable nucleophile(s) and α,β-unsaturatedcarbonyl by-product(s) derived from said polyol(s) duringpolymerization. The resulting possibilities are innumerable but whenbisulfite is selected to be said suitable nucleophile, and acrolein isselected as the α,β-unsaturated carbonyl, the addition products cancomprise sodium-3-propanal sulfonate, 1-hydroxy-2-propene-1-sulfonate,1-hydroxy-1.3-propane disulfonate.

Hydrogel adhesives polymerized in the presence of scavengers that arealso chain transfer agents, showed different material properties thanhydrogel adhesives polymerized without these scavengers. Further studiesrevealed that also chain transfer agents that are no scavengersinfluence the material properties of the polymerized hydrogel adhesive.Chain transfer agents that are scavengers are however preferred, due totheir benefit of residual monomer and impurity reduction.

The most important material properties are the rheological behavior andthe peel force. They are described in detail in EP 1025823 A1 and EP1025866 A1.

Typically the material properties are changed by varying the solidcontent of the monomer premix and/or the amount of crosslinker. This cannot easily be done, after the premix has been prepared. Adding chaintransfer agents is an easy and elegant way to optimize materialproperties without changing premix composition. This opens a way to amore flexible hydrogel production. It also saves costs if the premixdoes not have to be discarded, but the material-properties can bechanged by adding chain transfer agents.

In order to provide adhesives for secure initial and prolongedattachment and easy/painless removal the relation between the elasticmodulus and the viscous modulus as well as their dynamic behavior isalso of importance.

The adhesive has an elastic modulus at a temperature of 25° C. (770Fahrenheit) abbreviated G′₂₅ and a viscous modulus at a temperatur of25° C. (770 Fahrenheit) of G″₂₅.

The adhesive according to the present invention preferably satisfies thefollowing conditions;

-   G′₂₅ (1 rad/sec) is in the range 200 Pa to 30000 Pa.    -   preferably 500 Pa to 20000 Pa, most    -   preferably 1000 Pa to 10000 Pa.-   G″25 (1 rad/sec) is in the range 100 Pa to 30000 Pa.    -   preferably 100 Pa to 10000 Pa, most    -   preferably 300 Pa to 5000 Pa.        and the ratio of G″₂₅ (1 rad/sec)/G′₂₅ (1 rad/sec) (tan δ₂₅) is        in the range of 0.03 to 3. Preferred are tan δ₂₅-values between        0.2 and 0.9, more preferred between 0.4 and 0.8. Also preferred        are hydrogels with a tan δ₂₅-values above 1, more preferred        between 1.01 and 2, most preferred 1.02 and 1.5.

So far only values of tan δ₂₅ that are smaller than 1 have beendescribed. By the use of chain transfer agents it is now possible toobtain hydrogels with a ratio greater than 1. For some applications itcan be advantageous to have these values greater than 1.

The hydrogels described herein preferably have a 90° peel force on dryskin of between 0.3 to 5 N/cm, more preferably 1.5 to 3 N/cm. Peel forcecan also be measured at 180° on Polyethylene terephthalate (PET). Thehydrogels herein preferably have a peel force on PET of between 0.3 to5.0 N/cm, preferably between 0.5 to 3.0 N/cm and more preferably between0.8 to 2.0 N/cm. The methods for measuring peel force on skin and PETare described hereinafter in the test methods section.

Suitable chain transfer agents that are also scavengers include, but arenot limited to nucleophiles as stated above. Especially preferred issodium bisulfite.

Suitable chain transfer agents that are no scavengers include, but arenot limited to organic acids such as formic acid, acetic acid, ascorbicacid and the like, thiols, such as 2-mercapto ethanol, armomaticcompounds such as toluene, chlorobenzene, aniline, benzonitrile,anthracene and the like, halogenated compounds such as dichloromethane,chloroethanol and the like, polyalcohols and sugars such as glycerol,sorbitol, glucose, arabinose and the like, alcohols such as iso-propanolor n-propanol.

While particular embodiments of the present invention have beenillustrated and described, it would be obvious to those skilled in theart that various other changes- and modifications can be made withoutdeparting from the spirit and scope of the invention. It is thereforeintended to cover in the appended claims all such changes andmodifications that are within the scope of this invention.

Test Methods

1. pH of Monomer Solutions

The pH of a monomer solution can be measured using methods well known tothe art. For example, an lonlabph/ion level 2P meter can be usedequipped with a SenTix 41 electrode (available from WissenschaftlichTechnische Werkstaetten).

2. Residual NaAMPS and Acrylic Acid in Polymerized Hydrogels

Sample Preparation: 100 ml of 0.9% w/v saline solution are added to1.0000 g hydrogel and the mixture is shaken in a thermostatic bath for aminimum of 16 hours at 40° C. An aliquot of the exctract is collectedinto a syringe and transfered it through a 0.20 μm hydrophilic filterinto a HPLC autosampler vial.

Analysis: Reversed-phase HPLC/DAD, −50il of the hydrogel filtrate (asabove) is injected directly into the HPLC, for example an Agilent Series1100 equipped with an Agilent Series 1100 solvent delivery module,Agilent Series 1100 auto injector, Agilent Series 1100 photo diode arraydetector and an Agilent Zorbax SB AQ 4, 6×150 mm 5 lm analytic-columnand an Agilent Zorbax SB AQ 4, 6×12.5 mm as guard-coloumn. The mobilephase comprises 96% of eluent A (H2O, containing 0,867 mmol/l Phosphoricacid) and 4% of eluent B (Acetonitrile). The flow rate is 1, 2 ml/min.The analytic temperature is 30° C. A photo diode array channel 200 nm(bandwidth 5 nm) is used for detection, the UV Spectra across 190-300 nmcan be applied for peak purity assessment. The level of analyte isquantified using standard procedures well known to the art and reportedas micrograms analyte per gram of hydrogel (ppm). The quantitativedetection limit of NaAMPS is below 5 microgram analyte per gram hydrogel(ppm). The quantitative detection limit of Acrylic Acid is below 3microgram analyte per gram hydrogel (ppm), based on a signal/noise ratioof 10.

3. Residual Acrylonitrile and Acrolein in Polymerized Hydrogels

Sample preparation:

The protective foil is removed from the “Hydrogel-Sample”. Then c. 5 gare weighed into a wide-necked bottle. To the sample 500 ml ofNaCl-solution (0.9% w/w) are added. This preparation is stored at 40° C.for c. 24 hours. During normal working time the bottle is shakenvigorously every hour. After 24 hours the bottle is allowed to cool downto room temperature, then the liquid phase is separated.

Final determination:

Principle:

Acrolein and acrylonitrile are determined via purge & trap GC-MSanalysis. For purge & trap a suitable commercial autosampler can beused. The autosampler is connected to a capillary gas chromatographcoupled to a quadrupole mass spectrometer.

Off-line purge & trap can be carried out as well, then the adsorptiontube has to be analysed further on a GC-MS system equipped with athermodesorption unit.

Principle information about the analytical technique is given in EPAmethods 5030B and 8260B.

For quantification an external standard procedure is recommended.Standard addition method can cause systematic errors, if residualbisulfite is present in the extract, which may react with the spikedstandards. In such a case too high values are evaluated.

A portion of 5 ml (2 ml for higher concentrated or foaming sampleextracts) of the separated aquatic extract is used for purge & trapGC-MS analysis.

Possible measurement parameters are given below:

For purge & trap the autosampler PTA-3000 (supplied by IMT) was used:sample temperature: 40° C. purge time: 20 min purge flow: 20 ml He/minvalve temperature: 80° C. transfer line: 200° C. trap cooling −120° C.water trap −15° C. temperature: temperature: trap desorption temp.: 200°C. desorption 10 min time:

Chromatographic conditions:

fused silica column:

RTX-VMS (supplied by Restec) length: 60 m, internal diameter 0.32 mm,film thickness 01.8 μm

-   Temp.-Progr.: 7 min isothermal at 40° C.    -   40° C.-80° C. with 7 K/min    -   80° C.-220° C. with 14 K/min    -   13 min isothermal at 220° C.-   Injector temperature: 200° C. Transfer line temperature: 220° C.-   carrier gas: helium 0.6 bar-   Quadrupol MS system (e.g. MD 800 supplied by Thermo Quest)-   source temperature: 220° C.:-   ionisation: El⁺-   selected ion monitoring: m/z 52 and 53 for acrylonitrile    -   (m/z 53 used for evaluation)    -   m/z 55 and 56 for acrolein    -   (m/z 56 used for evaluation)

Calibration is carried out by preparing standard solutions in aNaCl-solution (0.9% w/w) at the interesting concentration level. Thestandard solution is analysed by purge & trap GC-MS under the sameconditions like the Hydrogel extracts.

4. Rheology

The rheology of hydrogels is measured at 25° C. using a HAAKE RHEOSTRESS1 oscillatory rheometer or the equivalent. A sample of thickness ofapproximately 1 mm and diameter of 20 mm is placed between two insulatedParallel Plates of 20 mm diameter, controlled at a temperature ofapproximately 25° C. using a Peltier system or equivalent. A DynamicFrequency Sweep is performed on the hydrogel in either stress or strainmode at an applied strain within the linear elastic response of thehydrogel (e.g., up to a strain of about 10%), with measurements atdiscrete frequency values between 47, 75 Hz (300 rad/sec) and 0, 143 Hz(0, 8992 rad/sec). Results are quoted as G′, G″ and tan delta atfrequency values of 1.0 and 100 rad/sec. The hydrogel is aged at least24 hours before measurement. The average of at least threedeterminations are reported.

5. Peel Force on Dry Skin

The peel force to remove hydrogel from dry skin is measured using asuitable tensile tester, for example an Instron Model 6021, equippedwith a 10N load cell and an anvil rigid plate such as the Instronaccessory model A50L2R-100. Samples are cut into strips of width 25.4 mmand length between about 10 and 20 cm. A non-stretchable film of lengthlonger than the hydrogel is applied to the reverse side of the hydrogelsample (e.g. the substrate side) using double sided adhesive. A suitablefilm is 23μ thick PET, available from Effegidi S.p.A., 43052, Colomo,Italy. For samples with release paper, the release paper is removedprior to applying the hydrogel to the forearm and then rolling it intoplace using a compression weight roller to prevent air entrapmentbetween hydrogel and skin. The roller is 13 cm in diameter, 4.5 cm wideand has a mass of 5 kg. It is covered in rubber of 0.5 mm thickness. Thefree end of the backing film is attached to the upper clamp of thetensile tester and the arm is placed below. The sample is peeled fromthe skin at an angle of 90 degrees and a rate of 1000 mm/min. Theaverage peel value obtained during peeling of the whole sample is quotedas the peel value in N/cm. The average of triplicate measurements isreported.

6. Peel Force on PET

Peel force to remove hydrogel from poly(ethylene teraphthalate) (PET)film is measured using a suitable tensile tester, for example a ZwickZ1.0/THIS, equipped with a 50N load cell and a pneumatic grip like ZwickModel: 8195.01.00 and attachment for a rigid lower plate, e.g. steel,oriented along the direction of cross-head movement. Freshly producedhydrogel is stored in a closed aluminium bag or similar for at least 12to 24 hours at room temperature before measuring. A defect free sampleof at least 10 cm in length Is cut from the hydrogel sample. A piece ofdouble sided adhesive, for example type Duplofol 020DIVB+L from LohmannGmbH Postoffice box 1454 56504 Neuwied, at least 130 mm long and 25.4 mmwide is stuck to the front side of the lower plate. The hydrogel ispunched out with a Zwick mechanical cutting press like Zwick model 7104using a cutting tool 25,4 mm wide and 25, 4 cm long. The second linderis removed from the tape and it is stuck on the back side of thehydrogel sample. A strip of standard PET of 23/thickness and no coronatreatment, is cut to about 300 mm×28 mm. Suitable material would include“Cavilen-Forex” from Effegidi S.p.A., Via Pro-vinciale per Sacca 55,I-43052 Colomo, Italy. The release liner is removed from the hydrogeland the bottom end fixed to the rigid plate by regular tape. Thestandard substrate is then applied onto the body adhesive using a handroller once forward and once backward at a speed of 1000 to 5000 mm/min.The roller is 13 cm in diameter, 4, 5 cm wide and has a mass of 5 kg. Itis covered in rubber of 0,5 mm thickness. The measurement is preferablyperformed within 10 minutes of application of the substrate.

The free end of the standard substrate is doubled back at an angle of180 degrees and the rigid plate is clamped in the lower clamp of thetensile tester. The free end of the standard substrate is fixed in theupper clamp of the tensile tester. The peel test is performed at a speedof 1000 mm/min. The initial 20 mm of peel is disregarded and the averageforce over the remaining length is quoted as the peel force in N/cm. Theaverage of triplicate measurements is reported.

EXAMPLES

General Description of Gel Preparation

a) Laboratory Samples Containing Na Amps

Approximately 22.4 parts of 50 wt % Na-AMPS solution, approx. 16.6 partsof acrylic acid and approx. 10.4 parts of deionized water are mixedtogether. To this solution approximately 5.5 parts 50 wt % NaOH is addeddropwise with constant stirring, while maintaining the temperature below30° C. with an ice bath. After addition of the NaOH approx. 44.8 partsof glycerol are added together with approx. 0.1 parts crosslinker (i.eIRR 210) and approx. 0.2 parts of photoinitiator (e.g Darocure 1173 orIrgacure 2959) and nudeophiles X (e.g. sodium bisulfite oraminoguanidine). The nucleophiles can be added as pure compounds or assolutions). The procedure is carried out in brown glassware which iscovered with a brown watch glass to protect the reaction mixture fromlight. After stirring for about 15 to 30 minutes the reaction mixture ispoured on a teflon coated plate to give a 1 mm thick layer. The reactionmixture is than irradiated with a 2000 W Hönle UV lamp at 100 mW/cm².Typical irradiation times range between 60 s to 180 s. The gels are thencovered with regular photocopy paper and peeled off the plate. The otherside of the gel is covered with a release liner (e.g. siliconizedpaper).

b) Laboratory Samples Non-Containing Na-AMPS

Approximately 57.8 parts of 50 wt % Na-Acrylate (70% neutralized)solution, approx. 41.9 parts of glycerol are added together with approx.0.1 to 0.3 parts crosslinker (i.e. IRR 210) and approx. 0.2 parts ofphotoinitiator (e.g. Darocure 1173 or Irgacure 2959) and nucleophile orchain transfer agent X (e.g. 2-Mercapto ethanol, formic acid or sodiumbisulfide). The compound X can be added as pure compound or as solution.The procedure is carried out in brown glassware which is covered with abrown watch glass to protect the reaction mixture from light. Afterstirring for about 15 to 30 minutes the reaction mixture is poured on toa teflon coated plate to give a 1 mm thick layer. The reaction mixtureis than irradiated with a 2000 W Hönle UV lamp at 100 mW/cm². Typicalirradiation times range between 60 s to 180 s. The gels are then coveredwith regular photocopy paper and peeled off the plate. The other side ofthe gel is covered with a release liner (e.g. siliconized paper).

c) Pilot Line Samples

The composition of the monomer mix is unchanged compared to thelaboratory samples (see a)). The addition of the nucleophiles X can bebatchwise into the stirred tank reactor or be online (e.g. staticmixer). The monomer mixture, including the nucleophiles, is extrudedonto a substrate (e.g a nonwoven webbing) at a basis weight ofapproximately 1.0 kilograms per square meter. Polymerization is carriedout by irradiating with UV light using 1 to 7 2000 W Hönle UV lamps or 1to 12 high power IST UV lamps or a combination of both. The lamps can beequipped with glass filters that cut wavelength below 320 nm. By thisprocess the monomer solution is converted into an adhesive hydrogel.After passing the UV lamps this adhesive hydrogel is covered with arelease liner (e.g siliconized paper or oriented polypropylene (OPP)foil), trimmed to the required width and wound up onto rolls.

d) Preparation of Nucleophile Solutions

The solutions are prepared by dissolving the nucleophiles in deionizedwater.

Experimental Results Acrylic acid AMPS Acrolein X (ppm) (ppm) (ppm)Aminoguanidine 0 ppm (laboratory) NA NA 1.135 Aminoguanidine NA NA 0.4351000 ppm (laboratory) NaHSO₃ 0 ppm (pilot line) 210 441 0.6 NaHSO₃ 500ppm (pilot line) 234 383 0.07 NaHSO₃ 1000 ppm (pilot line) 215 423 <0.05

The following table shows that the scavenger sodium bisulfite also actsas a chain transfer agent and influences the material properties. XAcrylic acid AMPS Acrolein G′₂₅ [Pa] G″₂₅ [Pa] Peel on PET (ppm) (ppm)(ppm) (ppm) (1 rad/sec) (1 rad/sec) tan δ₂₅ (N/in) NaHSO₃ 210 441   0.63374 1780 0.53 0.94 0 ppm NaHSO₃ 234 383   0.07 2592 1606 0.62 1.60 500ppm NaHSO₃ 215 423 <0.05 1654 1261 0.76 2.64 1000 ppm NaHSO₃ 89 26 not1394 1469 1.05 2.50 2000 ppm detected

In the following table the influence of a chain transfer agent that isno nucleophile (e.g. formic acid) on a laboratory sample containing noNaAMPS is shown. X Acrylic acid G′₂₅ [Pa] G″₂₅ [Pa] tan Peel on PET(ppm) (ppm) (1 rad/sec) (1 rad/sec) δ₂₅ (N/in) Formic Acid 1188 109274791 0.44 0.55 3200 ppm Formic Acid 1077 8975 4191 0.47 0.47 6400 ppmFormic Acid 870 7013 3679 0.52 0.56 12800 ppmPostinitiation by Pretreatment with Redox Couples

Residual monomers, impurities and by-products can also be reduced byadding a mixture of the compounds X,Y,Z to the monomer mix prior toUV-polymerization. The compounds X,Y are forming redox couples which areable to initiate polymerizations. These redox couples include e.g.Fe²⁺/OH₂O₂, Fe²⁺/NaPS. Iron complexing agents Z (e.g. BASF Trilonbrands) can be added in addition to the redox couples to (partially)complex the iron ions.

For the following table the acrylic acid was extracted for analysis atthe same day the samples were prepared: Acrylic acid X Y Z Extractedafter (ppm) Fe²⁺ H₂O₂ Trilon D 0 days 811 (0 ppm) (0 ppm) (0 ppm) Fe²⁺H₂O₂ Trilon D 0 days 586 (50 ppm) (3000 ppm) (0 ppm) Fe²⁺ H₂O₂ Trilon D0 days 481 (50 ppm) (3000 ppm) (12.5 ppm) Fe²⁺ H₂O₂ Trilon D 0 days 359(50 ppm) (3000 ppm) (25 ppm) Fe²⁺ H₂O₂ Trilon D 0 days 239 (50 ppm)(3000 ppm) (37.5 ppm)

The residual monomer reducing effect continues with time: Acrylic acid XY Z Extracted after (ppm) Fe²⁺ H₂O₂ Trilon D 0 days 481 (50 ppm) (3000ppm) (12.5 ppm) Fe²⁺ H₂O₂ Trilon D 4 days 403 (50 ppm) (3000 ppm) (12.5ppm) Fe²⁺ H₂O₂ Trilon D 7 days 269 (50 ppm) (3000 ppm) (12.5 ppm) Fe²⁺H₂O₂ Trilon D 14 days 14 (50 ppm) (3000 ppm) (12.5 ppm) Fe²⁺ H₂O₂ TrilonD 0 days 239 (50 ppm) (3000 ppm) (37.5 ppm) Fe²⁺ H₂O₂ Trilon D 4 days 24(50 ppm) (3000 ppm) (37.5 ppm) Fe²⁺ H₂O₂ Trilon D 7 days 10 (50 ppm)(3000 ppm) (37.5 ppm)

1. A process for making a hydrogel comprising 10-90 wt % water, 10-60 wt% of a cross-linked hydrophilic polymer made from at least one startingmonomer type, and 10-80 wt % of at least one polyol, wherein saidprocess comprises the steps of a) preparing a starting monomer solutionfrom 10-90 wt % water, 10-60 wt % of said starting monomer, and 10-80 wt% of said polyol, and adding a modifying compound to said monomersolution prior to polymerization of the so formed mixture, then b)polymerizing said monomer within a reaction medium comprising 10-90 wt %water, 10-60 wt % of said starting monomer, and 10-80 wt % of saidpolyol, in the presence of the modifying compound to form a hydrogel,wherein the modifying compound is selected from the group consisting ofa thiol, a sulfite, a metabisulfite, and a bisulfite.
 2. The process ofclaim 1 wherein the modifying compound is added directly to the monomersolution before the polymerization in a stirring vessel, a tube, or astatic mixer.
 3. The process of claim 1 wherein in addition to themodifying compound, a scavenger compound is added to the monomersolution.
 4. The process of claim 1 wherein in addition to the modifyingcompound, a chain transfer agent is added to the monomer solution. 5.The process of claim 1 wherein in addition to the modifying compound, ascavenger compound and a chain transfer agent are added to the monomersolution.
 6. The process of claim 1 wherein a residual monomerconcentration in the hydrogel below 10000 ppm.
 7. The process of claim 1wherein the polymerization of said starting monomer is conducted at a pH3.5 to
 7. 8. The process of claim 1 wherein said hydrogel comprises20-70 wt % water.
 9. A The process of claim 1 wherein adding themodifying compound comprises adding to said monomer solution anucleophile which reacts with said residual starting monomer, impurity,by-product, or mixture thereof by an addition reaction.
 10. The processaccording to claim 9 wherein said by-product comprises anα,β-unsaturated carbonyl produced from said polyol.
 11. The process ofclaim 10 wherein said polyol comprises glycerol.
 12. The process ofclaim 11 wherein said by-product comprises acrolein.
 13. The process ofclaim 9 wherein the bisulfite is present in an amount of less than 30000ppm with respect to the hydrogel product.
 14. The process of claim 1wherein polymerization of said starting monomer is conducted at leastpartly by UV irradiation.
 15. The process of claim 1 wherein saidreaction medium comprises a photoinitiator.
 16. The process of claim 15wherein said photoinitiator is selected from the group consisting of2-hydroxy-2-methyl-propiophenone,4-(2-hydroxyethoxy)-phenyl-(2-hydroxy-2-methylpropyl) ketone, Irgacure500, and 1-hydroxycyclohexyl phenyl ketone.
 17. The process of claim 16wherein said photoinitiator is used in said monomer solution at aconcentration less than 5 wt %.
 18. The process of claim 1 wherein thepolymerization is conducted by UV curing, and an integrated UV intensityat wavelengths less than 280 nm is less than 10% of the total integratedUV intensity with wavelengths less than 400 nm.
 19. The process of claim18 wherein said polymerization is carried out under a total UVA energyranging from 0.1-30 J/cm².
 20. The process of claim 1 wherein saidstarting monomer comprises acrylic acid.
 21. The process of claim 1wherein said hydrogel is adhesive.
 22. The process of claim 1 whereinsaid hydrogel has a tan δ₂₅ between 0.03 and
 3. 23. A hydrogel preparedby the process of claim
 1. 24. A hydrogel comprising 10-90 wt % water,10-60 wt % of a cross-linked hydrophilic polymer made from startingmonomer(s), and 10-80 wt % of a at least one polyol, said hydrogelprepared by polymerizing said starting monomer(s) in the presence ofsaid water and polyol(s), wherein said hydrogel contains less than 100ppb of α,β-unsaturated carbonyl by-product(s) derived from saidpolyol(s) during polymerization.
 25. The hydrogel of claim 25 where saidα,β-unsaturated carbonyl by-product comprises acrolein.
 26. (canceled)27. The process of claim 1 wherein the residual monomer concentration isbelow 500 ppm.
 28. The process of claim 1 wherein the residual monomerconcentration is below 10 ppm.
 29. The process of claim 1 wherein thepolymerization of said starting monomers is conducted at pH 4.5 to 6.30. The process of claim 9 wherein the bisulfite is present in an amountof less than 10,000 ppm with respect to the hydrogel product.
 31. Theprocess of claim 9 wherein the bisulfite is present in an amount of lessthan 1,000 ppm with respect to the hydrogel product.
 32. The process ofclaim 16 wherein the photoinitiator is used is said monomer solvate at aconcentration less than 1 wt %.
 33. The process of claim 16 wherein thephotoinitiator is used is said monomer solvate at a concentration lessthan 0.4 wt %.
 34. The method of claim 18 wherein the integrated UVintensity at wavelengths less than 320 nm is less than 1% of the totalintegrated UV intensity with wavelengths less than 400 nm.
 35. Thehydrogel of claim 24 wherein said hydrogel contains less than 20 ppb ofα,β-unsaturated carbonyl by-product(s).